In recent years, a light-emitting element which can provide high luminance by current flow to an organic compound has attracted attentions, like a light-emitting element including a light-emitting organic compound.
A basic structure of a light-emitting element is a structure in which a layer containing a light-emitting organic compound is interposed between a pair of electrodes. By applying voltage to the element, electrons from one of the pair of electrodes and holes from the other electrode are injected to a light-emitting layer, so that current flows. The electrons and holes (i.e., carriers) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light.
It is to be noted that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
Since light-emitting elements are usually formed by using a thin film with a thickness approximately on the order of submicron, it is a great advantage that the light-emitting elements can be manufactured to be thin and lightweight. In addition, since it takes the time on the order of microseconds or less to generate luminescence after carrier injection, it is also one of the features that the speed of response is quite fast. Further, since sufficient luminescence can be obtained at a direct-current voltage of several to several tens of volts, the power consumption is relatively low. From these advantages, light-emitting elements have attracted a lot of attentions as the elements for the next-generation flat panel displays. In particular, the light-emitting elements are expected to be applied to mobile devices and the like, taking advantage of the features of thinness, lightness in weight and the like.
Most of characteristics of light-emitting elements depend greatly on materials forming the light-emitting elements, and various materials have been developed aiming at the use for light-emitting elements. In addition, there is a possibility that even a material which has been developed as a material for a light-emitting element can be applied to various devices such as solar batteries or transistors, with use of characteristics of the material.
Light-emitting elements using such an organic compound need a material having an excellent carrier transporting property or injecting property, since a lot of organic compounds are poor in conductivity.
For example, Reference 1 discloses a composite material including an organic compound and an inorganic oxide which is intended to be used for a charge generating layer to be provided between light-emitting units in a light-emitting element having a plurality of light-emitting units. According to Reference 1, it is disclosed that a carrier transporting property or injecting property of the composite material is improved by an interaction between the organic compound and the inorganic oxide.
However, a film of the composite material disclosed in Reference 1 can be formed only by an evaporation method. There is a limitation on metal oxides which can be deposited by an evaporation method, and thus, the room for choices of materials is extremely small. In addition, as compared with a wet method typified by a spin coating method or an ink-jetting method, there is a concern that a film formation using an evaporation method has difficulty in treating with a larger substrate in a manufacturing process of elements, and disadvantage for industrialization (Reference 1: Japanese Patent Laid-Open No. 2003-272860).